1. Field of the Invention
The present invention relates to magnetic recording media such as a hard disk mounted on a computer, and the like, particularly to a magnetic recording medium which has a high coercive force and which inhibits the generation of noises during regeneration of a recording signal.
The present invention also relates to a method and apparatus for accurately measuring and evaluating a signal decay by the thermal fluctuation of a magnetic recording medium in a simple method before the magnetic recording medium is incorporated into a magnetic storage apparatus.
2. Description of the Related Arts
As this kind of magnetic recording medium, for example, such a magnetic recording medium as described in Japanese Patent Application Laid-Open No. 227516/1996 is suggested. This magnetic recording medium comprises a basic constitution wherein two Co—Pt-based magnetic layers are disposed on a substrate and a non-magnetic layer containing Cr and Mo as main components is interposed between these two magnetic layers. When the magnetic layer is divided into two layers with the non-magnetic layer in this way, noise can be reduced in replaying a recorded signal, because the film thickness of each magnetic layer is thinner than that of a single magnetic layer which is equal to the total film thickness of the three layers. On the other hand, for the purpose of reducing the noise, there is suggested a magnetic recording medium employing a constitution in which an inter-metallic compound of a B2 structure is used as an underlayer, as described in U.S. Pat. No. 5,693,426. This inter-metallic compound of the B2 structure is generally called a seed layer. In this case, the fine seed layer of the B2 structure is further formed as a lower layer under a Cr-based underlayer which is compatible with a Co-based magnetic layer, whereby the Cr-based layer which is the underlayer can be allowed to carry out epitaxial growth. In consequence, the fine Cr-based underlayer can be formed, and the magnetic layer present on the underlayer can also be allowed to do the epitaxial growth in accordance with the fine state of the underlayer, which permits the reduction of the noise. Such a type of magnetic recording medium can be suitably used as the magnetic recording medium for MR (magnetism-resistant type) heads in which the inhibition of the noise is more desired than the increase of an output.
In the magnetic recording medium comprising the constitution described above, the magnetic layers are divided with the non-magnetic layer, and hence each magnetic layer is surely thin and the particle size of crystals is small, whereby the noise can be inhibited. In addition, the particle diameter of the crystals of the magnetic layer can be decreased by the epitaxial growth of an upper layer due to the seed layer, whereby the noise can also be inhibited. However, if the crystals of the magnetic layer are made very fine for the purpose of the reduction of the noise, magnetization is thermally unstable, and a recorded signal attenuates with time and finally it disappears. Accordingly, as the fine structure of the medium suitable for high density recording, it is important that the fine particles are formed, a particle diameter distribution is uniformed, the dispersion of a particle size is minimized, and the formation of the extremely fine particles which are easily affected by thermal fluctuation is inhibited.
Moreover, as this type of magnetic recording medium, for example, a magnetic recording medium described, for example, in Japanese Patent Application Laid-Open No. 259418/1997 is proposed. The magnetic recording medium is formed by laminating a seed layer consisting of at least Al1-xCox, and the like, a Cr or Cr alloy under film, and a Co alloy magnetic layer in this order on a substrate, and a high coercive force and low noise are achieved.
For the high coercive force in the magnetic recording medium, the seed layer enhances the crystal orientation property of surface (110) in a body-centered cubic crystal (bcc) of Cr or Cr alloy as the under film, and the crystal orientation property of surface (100) is enhanced and achieved, in which the easy axis of magnetization (c-axis) of the Co magnetic layer epitaxially grown on the under film is parallel with the inside of the plane.
Moreover, in this magnetic recording medium, since the thickness of the under film can be reduced by disposing the seed layer, the Co magnetic particles on the thinned under film are formed to be fine, so that the magnetic transition region (magnetic domain wall width) between recording bits can be reduced, and noises can be reduced.
Moreover, when the crystal particle diameter of the magnetic layer becomes very fine so as to reduce the noises, the magnetization becomes thermally unstable, thereby causing a phenomenon in which recorded signals are attenuated with time and finally disappear, that is, a phenomenon called thermal fluctuation. The noise has a trade-off relationship with the thermal fluctuation. When the crystal particle diameter of the magnetic layer becomes fine, the noises are reduced, but the signal attenuation by the thermal fluctuation increases, and the recorded signals are attenuated or easily disappear with the elapse of time. When the thermal fluctuation occurs, in addition to the signal attenuation (readback output decrease), the medium noise increases, or PW50 (the half pulse width of an isolated readback signal) value is deteriorated.
As described later, as the fine structure of the medium preferable for high-density recording, with the attaining of the fine magnetic layer crystal particles, it becomes important to reduce the dispersion of the particle size (particle diameter distribution) and to depress the generation of the excessively fine particles susceptible to the influence of the thermal fluctuation.
However, when the film thickness of the seed layer is increased to obtain a high coercive force, the crystal particle diameter and particle diameter distribution forming the seed layer increase, and the crystal particle diameter and particle diameter distribution of the under film and magnetic layer formed on the seed layer also increase with the crystal growth, thereby causing a problem that the noise reduction cannot be realized.
On the other hand, in recent years, the computer performance enhancement has been advanced, and the amount of handled information has rapidly increased. Accordingly, the capacity of the magnetic recording apparatus, particularly a hard disk drive has increased steadily.
For the hard disk drive, in recent years, with the enhancement of the recording density, the phenomenon called as the thermal fluctuation has come into question. In this phenomenon, the signal written to the magnetic recording medium is attenuated with the elapse of time. The cause for this is said to be that the magnetization becomes thermally unstable by the fine division of the magnetic particles. Therefore, this attenuation is particularly remarkable under a high-temperature environment. Therefore, in order to evaluate the magnetic recording medium, the above-described signal attenuation needs to measured under the high-temperature environment to evaluate whether or not the thermal fluctuation properties are satisfactory.
However, in a conventional spin stand type electromagnetic conversion property evaluating apparatus, since the apparatus is large scaled for the volume of an environmental tank, it is difficult to place the magnetic recording medium in a high-temperature state.
Moreover, an off-track phenomenon called a thermal off-track occurs during the measurement under high temperatures, in which a head deviates from a track by the thermal expansion of a head suspension. Since the signal attenuation occurs with the occurrence of the thermal off-track, it is difficult to accurately measure and evaluate only the signal attenuation by the thermal fluctuation of the magnetic recording medium.